In recent years, cardiac wall motion analysis has been put to practical use by implementing two- or three-dimensional speckle tracking (ST) technology on moving images of ultrasound images. In a commonly-used ST process, contours of the inner and the outer layers of myocardia in a temporal phase at end-diastole (the first R-wave phase) or end-systole are provided as initial contour positions. Further, during the ST process, contour positions in all the necessary temporal phases are obtained by automatically tracking the initial contour positions in the remaining temporal phases, while using movement information (motion vectors) obtained by performing a local pattern matching process or by implementing an optical flow method. For this reason, to realize accurate tracking of the contour positions, it is essential to estimate the motion vectors accurately.
In an early diastolic (e′) phase or a systolic peak (s′) phase where the strain rate of the heart is large, however, extent of pattern change among frames or among volumes is large. For this reason, when using the speckle tracking process, it is difficult to accurately estimate the motion vectors if the frame rate or the volume rate is insufficient. In particular, when two-dimensional images are used, the tracked contour is influenced by a movement of going through a scanned cross-sectional plane (which is called a “through-plane” movement). Therefore, it is more difficult to accurately estimate the motion vectors, because the extent of pattern change among the frames is even larger.
Further, when the image quality of the moving images is low due to noise or artifacts, it is difficult to accurately estimate the motion vectors in such a part where the unnecessary components are mixed in. When it is not possible to accurately estimate the movement information for any of the various factors described above, a tracking failure occurs, and as a result, it is not possible to accurately perform the wall motion analysis.
To cope with this situation, various methods for performing an accurate tracking process have been proposed. However, none of the proposed methods is able to address all of the various factors described above. Thus, with the currently-used speckle tracking methods, it is not possible to accurately obtain contour tracking results in some situations. Further, the problems described above similarly occur when a region of interest is tracked by using moving images of medical images that are other than ultrasound images.